Move {leading,trailing}WhitespacePosition to editing/ from dom/

This patch move {leading,trailing}WhitespacePosition to editing/htmlediting.cpp
from Position.cpp to reduce editing/ dependency from dom/

BUG=n/a
TEST=n/a; no behavior change

Review URL: https://codereview.chromium.org/344793012

git-svn-id: svn://svn.chromium.org/blink/trunk@176736 bbb929c8-8fbe-4397-9dbb-9b2b20218538